1. Field of the Invention
The present invention relates to a method for eliminating mercury from liquids by amalgam formation with silver and to silver coated fiber for used in the method.
2. Discussion of Background
Due to the high toxicity of mercury, many industrial processes require the elimination of mercury from liquids used in or resulting from the process.
For example, in the production of aqueous alkali metal hydroxides according to the amalgam process, or the preparation of alkali metal alcoholates by reacting alcohols with alkali metal amalgam, solutions are obtained which contain significant levels of mercury. In order to further use these solutions, the mercury must often be eliminated, especially in those cases where the solutions are to be used in food or pharmaceutical applications.
Conventional methods of eliminating mercury from solutions obtained during the preparation of aqueous alkali metal hydroxides primarily involve removal of the mercury by mechanical methods, such as filtration using special carbon filters or centrifuging. While these methods allow the reduction of mercury to a residual level of 0.5 to 1 ppm, this is still insufficient for many applications.
Another conventional process removes mercury from the mercury-contaminated alkali or alcoholate solutions of the above mentioned processes by passing the contaminated solutions over large silver surfaces. During passage over the silver surface, the mercury becomes bound as silver amalgam.
In order to implement the above conventional process in industry, the liquids to be purified flow through pipes or columns which are filled with silver in a suitable form. Obviously, these columns must be highly effective in terms of mercury elimination, and at the same time achieve as high as possible a flow rate per unit time per unit column volume. Thus the mercury elimination efficiency depends on the silver surface area available per unit volume of the column and on the contact time of the liquid with the silver surface.
U.S. Pat. Nos. 4,230,486 and 4,353,741 disclose the use of silver powder, silver granules or silver-coated particles in the above-mentioned types of mercury removal columns. The supports described for the silver include powdered or granular substances made from glass, brick, activated carbons, refractory materials or grinding materials.
However, the efficiency (effectiveness of removal of mercury relative to throughput of liquid being purified) of the mercury elimination using powdered or granular silver materials according to the prior art is significantly limited. While selecting small particles, such as fine powders, does provide a large silver surface area, these same fine powders create extremely high flow resistance to the liquid being purified, such that the flow rate per unit time and unit column volume decreases to the point of making the process uneconomical.
Alternatively, finely porous material, such as activated carbon, has been used as a support for the silver. While activated carbon is known to have a very large internal surface area, it unfortunately offers no advantages in this case since during silvering, the fine porous structures are closed up by silver, making the initially large internal surface area unavailable and ineffective. Thus only the external surface area of the carbon particle, in effect, remains active. Even if small channels remain in the carbon, these become useless because the flow resistance through them is far too large due to capillary forces. A further serious disadvantage to silver-coated activated carbons is their tendency towards explosive decomposition.
Thus a method is needed which provides for removal of mercury from liquids at high levels of effectiveness, while maintaining high levels of throughput of the liquid being purified.